1. Field of the Invention
The present invention relates to Cassegrain aerial systems, that is to say aerial systems of the type comprising a relatively large main reflector facing a relatively small sub-reflector. The invention relates particularly to small Cassegrain aerial systems in which the maximum dimension of the main reflector is comparable to the wavelength of the radiation employed--of the order of ten wavelengths or less for example.
2. Description of Related Art
A problem which arises in all Cassegrain aerial systems, and which becomes particularly acute when the mainreflector is less than ten wavelengths in diameter, is interference between the sub-reflector and the main reflector beam, in both reception and transmission. This can be reduced by reducing the size of the sub-reflector, but if the sub-reflector is too small, then radiation which would otherwise have been reflected from the sub-reflector spills over around its edges and interferes with the main reflector beam, resulting in high side lobes and a consequent loss of directivity. It will be appreciated that "spillover" is liable to arise in both reception and transmission. Interference and spillover, being wavelength-dependent, tend to severely limit the bandwidth of the aerial system.
Considering the case of transmission only, for the sake of simplicity, a satisfactory performance can only be achieved by utilising a small sub-reflector fed by a correspondingly narrow beam of radiation (which has a frequency typically of the order of several GHz). A conventional feed-horn is not suitable for generating this narrow radiation beam because it would need to be of comparable diameter to the sub-reflector and would therefore be liable to cause interference. Instead a dielectric aerial (or "polyrod") has been used, which consists of a rod of polythene or other suitable dielectric material extending from a conventional tubular waveguide towards the sub-reflector. The polyrod acts as a leaky waveguide, so that its radiation pattern is determined essentially by the length of the portion which extends from the tubular waveguide, and depends only weakly on its diameter, in accordance with a formula given on page 37 of the book "Dielectric Aerial" by D. C. Kiely (published by Methuen & Co.). This book is hereby incorporated by reference.
A narrow beamwidth can thus be obtained from a small-diameter polyrod. However, the sub-reflector must be maintained in accurate alignment with the polyrod in order to achieve a satisfactory performance. In order to maintain accurate alignment of the polyrod and sub-reflector, particularly under conditions of high acceleration, fairly substantial supporting struts are required. However the struts tend to interfere with the radiation beam emerging from the sub-reflector.
In response to this essentially mechanical problem, the splashplate was developed. This consists of a spigot expanding into a generally conical portion, on the base of which conical portion a metal film is deposited to form the sub-reflector. Thus the sub-reflector is supported entirely by its dielectric feed, and, since it is integral with its dielectric feed, no misalignment can arise. The aerial performance achieved represents the current state of the art.